The standard options are "titlecase," "sentencecase," "uppercase," and "lowercase." Various abbreviations and marks for these are used in proofreading. It's somewhat specialized vocabulary because outside of publishing you rarely have cause to say things like, "this word needs to be lowercased."

(Reread the answer and changed my comment) I think you are meaning those four words as verbs, but it is not obvious from your comment. I first read them as descriptions and posted that your answer did nothing to answer the question.
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Colin FineMar 1 '11 at 12:37

Colin - I work in a publishing house. We say things like, "lowercase that," "titlecase here," etc. As MS Word lists these in its "Change Case" tool, I think of them as written out as single words, but generally we mark them with abbreviations on manuscript, e.g., "l/c" for "lowercase." These can be used as both adjectives and verbs as needed by circumstance.
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The RavenMar 3 '11 at 21:45

If being proper / correct is not your greatest concern.. Say you simply wanted your listener to "Re-write this sentence withOUT Capital letters."… I'd wager a bet that asking them to decapitalize it - would give you as good a result - as any.

If a capitalized word is a word whose first letter is in uppercase/majuscule and any following letters are in lowercase/miniscules, then the opposite of that must be a word whose first letter is in lowercase/miniscule and any following letters are in uppercase/majuscules.

We have no term to describe that.

“WASHINGTON” is in all uppercase.

“washington” is in all lowercase.

“Washington” is capitalized.

“wASHINGTON” is . . . in all uppercase save for its first letter, which is in lowercase.

For various reasons, Unicode defines three cases, not two; the third case is called titlecase, and it is specifically designed for the casing of the initial letter in a capitalized word. That is, one with all lowercase letters save the first one. This doesn't make any especial sense in English in a way graphically distinct from an initial-uppercase letter:

lowercased: little big horn

titlecased: Little Big Horn

uppercased: LITTLE BIG HORN

But in other languages and scripts it can make a difference:

lowercased: ᾲ στο διάολο

titlecased: Ὰͅ Στο Διάολο

uppercase: ᾺΙ ΣΤΟ ΔΙΆΟΛΟ

Notice how in the first word (the article), the titlecased version has an iota subscript, but the all-uppercase version has the iota as a capital letter. That is because when converting to all-caps, you have to change the iota from its diacritic form to its letter form. It is unclear how much this really gets used or needed in Greek, but it is in the formal rules. You can see why it doesn’t arise in English.